Electric power distribution methods and apparatus

ABSTRACT

A plurality of end-user locations are served by a commercial utility grid. More than one and less than all of the end-user locations are themselves interconnected by a feeder, the feeder not metallically connected to the utility grid. The end-user locations each have a local AC bus that is not metallically connected to the utility grid or to the feeder, but that is linked by a coupler to both the utility grid and to the feeder. None of the local AC buses or the feeder is required to have the same phase or frequency as the utility grid. Locally generated electric power may be passed by means of the feeder to other end-user locations that are on the feeder. Each local AC bus has two or more inverters powering the bus.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional of U.S. application Ser. No.12/519,507, filed Dec. 12, 2008, which claims the benefit of U.S.application No. 60/996,954 filed Dec. 12, 2007, which applications arehereby incorporated herein by reference for all purposes.

BACKGROUND

Much attention has been given in recent times to energy policy andenergy conservation generally, and to electric power generation andtransmission in particular. Traditional models for power generation anddistribution may once have been the only workable ways to deliver powerto end users. But a variety of factors including political events,rising energy costs, technological progress, and concern for theenvironment have drawn attention to the need for new paradigms andapproaches.

It is apparent that one important goal is to make it possible to drawupon distributed energy resources as a source of electric power for endusers. There are, however, a number of forces seemingly conspiring tolimit or frustrate the use of distributed energy resources. One problemis that the operators of commercial power grids refuse to permit largeamounts of power to be fed into the grid from end-user locations.Typically the operator of a commercial power grid will permit feeding ofsuch power only up to a very small percentage of the distributioncapacity of the grid. This means that if one end-user location happensto have a large amount of locally generated power available, it islikely to be difficult or impossible to use the commercial power grid asa way to distribute that power to other end user locations.

It would thus be very desirable if a workable approach could be foundfor passing large amounts of power from one end-user location toanother, despite the lack of cooperation on the part of the operator ofthe commercial power grid.

As will be discussed in more detail below in connection with theinvention, experience reveals that moving electric power from one partof an end-user location to another, or from one end-user location toanother, in an efficient and reliable way, is not easy. Traditional waysof passing power from one grid to another have many drawbacks. In morerecent times, inverters have been developed that convert DC to AC in anefficient fashion and with improved quality of alternating current. Butthe inverters, taken singly, do not serve the end users as well as mightbe desired.

It would be very helpful if an approach could be found for coordinatingthe frequency and phase of generated AC so as to avoid conflicts amongthe two or more inverters that might be connected to a given AC powerbus.

Patents of possible background interest include U.S. Pat. No. 7,145,266to Lynch, et alia entitled Parallel-connected inverters with separatecontrollers having impedance current regulators, U.S. Pat. No. 7,116,010to Lasseter et alia, entitled Control of small distributed energyresources, and U.S. Pat. No. 6,693,409 to Lynch, et alia entitledControl system for a power converter and method of controlling operationof a power converter.

SUMMARY OF THE INVENTION

A plurality of end-user locations are served by a commercial utilitygrid. More than one and less than all of the end-user locations arethemselves interconnected by a feeder, the feeder not metallicallyconnected to the utility grid. The end-user locations each have a localAC bus that is not metallically connected to the utility grid or to thefeeder, but that is linked by a coupler to both the utility grid and tothe feeder. None of the local AC buses or the feeder is required to havethe same phase or frequency as the utility grid. Locally generatedelectric power may be passed by means of the feeder to other end-userlocations that are on the feeder. Each local AC bus has two or moreinverters powering the bus. The inverters for a particular local AC busare linked by out-of-band signaling with a latency far shorter than theperiod of the AC power, and at any given moment one of the inverters isa master and the others are slaves, so far as voltage, frequency, andphase are concerned. The DC buses powering the inverters are themselvesisolated from each other. The linkage from each inverter to its AC busis free from any transformer.

DESCRIPTION OF THE DRAWING

The invention will be described with respect to a drawing in severalfigures, of which:

FIG. 1 shows a small-town distribution system;

FIG. 2 shows a plurality of inverters powering a local AC bus;

FIG. 3 shows a detail of a bank of inverters, with particular attentionto out-of-band control links;

FIG. 4 shows detail of an inverter;

FIG. 5 shows an internal power circuit for a building;

FIG. 6 shows a prior-art standard interconnection;

FIG. 7 shows an installation with an independent grid for a singlebuilding; and

FIG. 8 shows an installation with multiple independent grids, as well asan unsynchronized feeder.

Where possible, like reference numerals have been used among the figuresto denote like elements.

DETAILED DESCRIPTION

FIG. 1 shows a small-town distribution system. In an exemplaryembodiment, AC power is transmitted to the area by a utility companyomitted for clarity in FIG. 1. The power is transmitted to the area byline 11 which may be 115 kilovolts (kV) three-phase AC power. This poweris reduced in voltage at a substation 24 operated by the utility companyfor distribution on a distribution line or grid 12 which may carry 13kV. This distribution grid is likewise three-phase AC power. Residences13 are served by the distribution grid 12, as is a medium industrialplant 16, small industries 17, 18, and 19, a shopping mall 25A.

It may be that the utility company will place a strict limit on theamount of electrical power that any one customer or end user ispermitted to generate locally under circumstances that might lead topower being fed back into the utility grid. Such a strict limit may inpart be justified by legitimate engineering concerns, and may be in partcaused by mindsets dating from earlier decades when all aspects of theutility grid were centrally planned and controlled.

Thus for example in a prior-art system, if the shopping mall 25A were topossess a source of on-site power 21, and if the connection of theon-site power through line 15 were such that power could be fed backinto the utility grid 12, there might be strict limits on the permittedgeneration capacity of the on-site power 21. The limit may be only asmall percentage of the power distribution capacity of the local grid12. In such a prior-art environment, there are many social,environmental, and economic benefits that might have been available ifonly the on-site power 21 could have been larger in its power generationcapacity than the capacity permitted by the utility or regulatoryenvironment. Such social, environmental, and economic benefits are,however, completely forgone in a prior-art system.

In one embodiment 41 of the invention, feeders 14 are provided.Independent area 42 is defined by a respective feeder 14 which connectsend-user locations 16 and 17. Note that the end-user locations 16 and 17represent more than one but less than all of the end-user locationsserved by the utility, and represent more than one but less than all ofthe end-user locations served by the distribution grid 12. As will bedescribed in more detail below, the presence of the feeder 14 permits alocal distribution of power from an independent power plant 22, 23. Thefeeder 14 is not metallically linked to the distribution grid 12, andneed not be operating at the same phase, or at the same frequency, asthe power provided by the utility via grid 12.

Each end user 16, 17 has a respective local AC grid omitted for clarityin FIG. 1. At each local AC grid is a coupler coupling the grid 12thereto, and a coupler coupling the feeder 14 thereto; the couplers arelikewise omitted for clarity in FIG. 1.

Independent area 43 is also defined by a respective feeder 14 whichconnects end-user locations 25A, 19, and 18. Note that the end-userlocations 25A, 19 and 18 represent more than one but less than all ofthe end-user locations served by the utility, and represent more thanone but less than all of the end-user locations served by thedistribution grid 12. As will be described in more detail below, thepresence of the feeder 14 permits a local distribution of power fromon-site power generators 20, 21. Just as was described in connectionwith independent area 42, the feeder 14 of independent area 43 is notmetallically linked to the distribution grid 12, and need not beoperating at the same phase, or at the same frequency, as the powerprovided by the utility via grid 12.

Each end user 25A, 19, 18 has a respective local AC grid omitted forclarity in FIG. 1. At each local AC grid is a coupler coupling the grid12 thereto, and a coupler coupling the feeder 14 thereto; the couplersare likewise omitted for clarity in FIG. 1.

As will be described in more detail below, the embodiment 41 offers manybenefits when compared with prior-art power distribution systems. Alocal power source 22, 23, 21, 20 can be of much greater powergeneration capacity than would be feasible in a prior-art system.Particular end users are able to enjoy greater reliability (e.g.up-time) with respect to the entirety of their energy needs, as comparedwith prior-art systems. The benefits that flow from generation of powerlocally to where it is needed, as compared with the legacy approach ofgeneration power at great distances from where it is needed, may befully enjoyed. The embodiment 41 may, under some circumstances, alsopermit an end-user to save money as compared with purchasing most of itspower from the utility.

It will be appreciated that while many embodiments of a system such asthat discussed in connection with FIG. 1 will be embodiments where thedistribution grid 12 is operated and powered by a commercial electricalpower utility, the teachings of the invention and its benefits are notlimited thereto. For example, the grid 12 might be a grid operated by amilitary unit, perhaps up to the size of a division, in an isolated areasuch as a desert area.

The system 41 of FIG. 1 will now be described from a differentperspective, namely that of an expanded power distribution system withseveral independent grids 14.

The architecture and control features of the independent grid 14 allowfor an almost unlimited ability to expand and integrate with existinggrids 12 as future growth desires.

The figure shows a small-town distribution system 41 that has been addedto with small on-site power 21, 20 and larger independent power units22, 23. The existing distribution system 12 could not supportinterconnection of these generators onto the distribution side of thetransformer 24.

By adding the power converters and control scheme, the power can beintegrated into the existing grid and supplied to the end users withoutthe need for upgrades to the existing grid 12.

Further expansion could include larger independent grids or new onesconnected through more power converters to the existing grid or to theother independent grids or both.

FIG. 2 shows an end-user system 40. The end-user system may represent anentire building, or may represent a load-panel area within a largebuilding. In some cases the end-user system 40 may represent a pluralityof buildings nearby to each other.

The system 40 comprises a plurality of inverters 29, 32, 33, 37 poweringa local AC bus 26. Each inverter has a line 28 connecting to the bus 26.The bus 26 has local loads 27.

The system 40 receives utility power from line 12, which may be steppeddown locally by means of transformer 30. For example the utility mayprovide 13 kV on the line 12, stepped down to three-phase 480 V AC bymeans of the transformer.

In day-to-day operation, the 480 VAC power from the utility is coupledby means of coupler 29 to local AC bus 26, and thence to local loads 27.

Importantly, however, there may be one or more local power generation orstorage devices forming part of system 40. As one example there may bean engine and generator or alternator 34, generating AC power which iscoupled by coupler 33 to the local bus 26. Importantly this power source34 can be very large, much larger than what would be permitted in aprior-art system. A portion of the power (the portion permitted by theutility) can be fed back into the grid 12 by means of coupler 29.

What will also be appreciated is that spare power in the local bus 26can also be fed into feeder 14 to be made available to other end users.This takes place through coupler 32 and optional transformer 31, whichmay for example step up locally generated power at 480VC to a 13 kVlevel which is more suitable to distribution to the other end users, whomight be a kilometer distant. (The higher voltage reduces resistivelosses in the feeder.) The amount of power that can be shared from oneend-user location to the next (using feeder 14) is far, far greater thanthe amount of power that could be shared if the only sharing mechanismwere that of the utility grid 12. It is difficult to overstate thebenefit of what has just been said about the feeder 14 when comparedwith a prior-art system where the only sharing opportunity is the legacygrid 12 operated by a legacy utility that may be suspicious of end userswho wish to generate large amounts of power locally.

In FIG. 2 we see examples of other equipment that might beinterconnected with the local bus 26. For example a DC storage system38, 39 permits storing DC energy received from the local bus 26, orstoring DC energy received from generation facility 35. Generationfacility 35 may be a cogeneration facility providing waste heat to someother process while generating power to be delivered to the local bus 26(and/or to storage 38, 39).

Stated differently, FIG. 2 shows a proposed building power distributionsystem 40, which has been made into an independent grid 26,unsynchronized with the main existing grid 12. There is power comingfrom two different, unsynchronized external feeds 12, 14, as well as twoonsite generators 34, 35 and incorporating energy storage 38, 39 in oneof them.

The inverters 29, 32, 33, 37 are controlled in such as way as to feedthe 480 VAC main (local) bus 26. In this way isolated, independent DCsources are made to behave like a single, larger voltage source feedingthe bus 26.

Separate system controls allow for the net energy supply to be variedbetween the isolated units 29, 32, 33, 37 to allow for more economiccontrol of energy sources and to improve redundancy.

In the above arrangement, the DC link voltages of the various inverters29, 32, 33, 37 are not connected together. This increases redundancysince the failure of a single DC link will not cause the failure of anyof the others.

It will be appreciated that while many embodiments of a system such asthat discussed in connection with FIG. 2 will be embodiments where thelocal AC bus is a three-phase “wye” grid, the teachings of the inventionand its benefits are not limited thereto. For example, the local C busgrid 12 might be a single-phase AC bus, or might be a “delta”three-phase system.

In the event of failure of the system 40, it is possible to restoreservice to the local bus 26 by opening breakers 28 and closing a bypassor transfer switch 25. In an exemplary embodiment, these steps arecarried out manually, and it is anticipated that these steps would berequired only very infrequently.

In a system of which FIG. 2 is an example, there is no limit to thenumber of different power sources. While FIG. 2 shows four possiblepower sources, there could be more. Typically there would be N+1 or N+2redundancy. The internal building loads 27 are the same as they would befor a standard utility power distribution layout.

The key to controlling the layout 41 of FIG. 2 is that all the inverters29, 32, 33, 37 that are tied to the main AC bus 26 must share the load27.

One embodiment being described herein is a power system which includes autility grid providing AC electric power connectivity and extendinggeographically to a plurality of first end-user locations and to aplurality of second end-user locations. The first end-user locationscomprise more than one and less than all of the end-user locations ofthe utility grid. Each of the first end-user locations comprises a localbus providing AC electric power connectivity. Each local bus hasassociated with it at least one AC load metallically connected theretoand drawing power therefrom. Each local bus is coupled by means of arespective first at least one coupler to the utility grid.

The at least one coupler comprises a first power-supply-inverter and asecond power-supply-inverter, each power-supply-inverter having an ACinterface and a DC interface and a controller. Eachpower-supply-inverter is disposed in a first mode to receive DC powerreceived at the DC interface and to generate AC power delivered at theAC interface, and is disposed in a second mode to receive AC powerreceived at the AC interface and to generate DC power delivered at theDC interface. The DC interface of the first power-supply-inverter iselectrically connected to the DC interface of the secondpower-supply-inverter.

The at least one coupler has a first AC interface defined as the ACinterface of the first power-supply-inverter and a second AC interfacedefined as the AC interface of the second power-supply-inverter. Thecontroller of the first power-supply-inverter and the controller of thesecond power-supply-inverter coupled so as to prevent the firstpower-supply-inverter and the second power-supply-inverter from being inthe first mode simultaneously for extended intervals, and so as toprevent the first power-supply-inverter and the secondpower-supply-inverter from being in the second mode simultaneously forextended intervals. By “extended intervals” we may mean in excess of afew milliseconds or in excess of half a second.

Each power-supply-inverter is disposed when in its first mode togenerate its AC power delivered at its AC interface consistent involtage and phase and frequency with any AC power present external tosaid AC interface.

FIG. 3 shows a detail of a bank of inverters 44, 46, 48, 50, withparticular attention to out-of-band control links 52, 53.

FIG. 3 is intended to portray a basic power layout, ignoring for themoment the source of DC power to the inverters. In this embodiment theAC grid (local bus) 26 is a four-wire system with Neutral line 57, feddirectly by the inverters 44, 46, 48, 50 without a transformerdownstream of them (between the inverter and the local bus 26).

It will be appreciated that a control scheme is required such that allof the inverters that are sourcing both kW and kVAR current into thesame voltage node (here, the local bus 26) will work together and notfight each other for control. The control scheme used in this system tosolve this problem works by having one unit, called the master, maintainthe voltage of the four-wire 480/277 VAC bus. Its individual phasecurrents are measured and the values passed along a high speedcommunication link 52 to the slave units to be copied. For example at aparticular moment, the master might be inverter 46 having respectivecontroller 47, while the other controllers 45, 49, 51 are acting asslaves, each controlling its respective inverter 44, 48, 50.

Each individual inverter 44, 46, 48, 50 still maintains over-current andother protection features, and a main breaker 56 can be remotely trippedby the system or inverter controller under certain conditions.

A standard prior-art way of paralleling a group of synchronous rotatinggenerators gives oscillations because they are trying to share power andspeed but can only be controlled by a mechanical fuel throttle that hasa slow reaction time between throttle change and output power change.There are no power electronics in such a system. The power electronicconverters of the present embodiments can change power at least 500 to1000 times faster.

The method for providing maximum fault current as fast as possibleinvolves the use of measured voltage limits by each individual inverter,such that if a voltage is out of range due to a fault, the maximumamount of current is sourced by each unit for as long as possible oruntil the fault is cleared.

The fault current must be sourced to open a distribution breaker. Thisis a difficult inverter challenge while being controlled in acurrent-sharing manner. See FIG. 5 which shows an internal power circuit73 for a building. Within the building is a 480 VAC main bus 71. Branchfeeders 72, 74, 76, 78 deliver AC power to circuits throughout thebuilding. Independent 800 VDC supplies 81, 82, 83, and 84 are shown,each able to deliver AC power to the main bus 71.

Consider what happens if there is a fault 85 giving rise to a faultcurrent 86. The inverters 81, 82, 83, 84 must be able collectively tosource enough fault current to open the protection on the faulted feederwithout taking down the whole independent grid.

One of the biggest limitations of inverter based microgrids is theirinability to be retro-fitted to existing buildings without completelyre-doing the breaker scheme because the inverters cannot source enoughfault current to trip a breaker. Thus the inverters will trip off(instead of a breaker being tripped) and the whole building goes dark,because the inverters have tripped off. This contrasts with a simplerprior-art system in which a fault current would have blasted open thebreaker of one of the branch feeders and the rest of the building wouldhave stayed on.

With a single, low impedance voltage source it is a simple calculationto determine how much fault current can be sourced versus how much isneeded for certain breakers. If the breaker is too big for the inverter(that is, if the tripping current for the breaker exceeds thecurrent-sourcing capability of the inverter), then the breaker needs tohave intelligent fault sensing controls added, or else the problem ofthe whole building going dark comes back.

With parallel inverters, what is needed is for the inverters to act morequickly than they could in a simple current-following mode. In anexemplary embodiment, the inverters are configured to sense a faultthrough voltage sensing, and then each inverter is released to feedmaximum current until the voltage comes back, which happens after thebreaker feeding the fault opens.

FIG. 5 also shows optional power sources 90, 91 which may provide ACpower to the AC bus 71. For example an inverter 91 may receive powerfrom (say) a photovoltaic array. Induction generator 90 may receivepower from a source of rotary energy such as a turbine. As will bediscussed further below, each of these sources is necessarilysynchronous to whatever voltage source defines the AC voltage on the ACbus 71. Here, the voltage source is the one or more inverters 81, 82,83, 84. What may happen, and what is in fact not uncommon, is that thepower from one or more of these sources may be of poor quality. Thesource may only deliver power to one phase. Even if the source deliverspower to all three phases, it may not be in perfect phase relationship.Loads that are assymmetric (as between the three phases) or thatintroduce power-factor loads may be present on the AC bus 71, and ifthey do, this may be beyond the ability of the sources 90, 91 to corrector compensate for. As will be discussed below, however, with suitableconfiguration the inverters 81, 82, 83, 84 can sample the voltagewaveforms present on the three phases, and can nearly instantaneouslydeliver voltage to the phases in such a way as to overcome nearly allsuch problems. In this way, an end user of a system 73 can make use ofcommercial, off-the-shelf power sources even if they produce power ofpoor non-utility-grade quality. In an exemplary embodiment, the combinedpower generation capacity of the non-utility-grade sources 90, 91 mightbe up to fifty percent of the combined power generation capacity of theinverters 81, 82, 83, 84. In another embodiment the percentage might besixty-five percent.

To recap, in the layout of FIG. 4 above, the DC link voltages of thevarious inverters are not connected together. This helps reduce faultcurrent levels and increases redundancy. The output AC inverters arestill able to work together as a single unit voltage source by havingthe transistor PWM (pulse-width modulation) pattern sent from a mastercontroller (one of 45, 47, 49, 51) and passed through to the slavecontroller (the others of 45, 47, 49, 51) of each individual invertercontrol.

The Master PWM Controller acts as it would act if it were driving asingle inverter, adjusting the PWM pattern to maintain a fixed voltageand frequency no matter what the load. This can include adjusting thePWM pattern to compensate for non-linear load characteristics that wouldotherwise cause voltage distortion, thus actively filtering harmoniccurrents.

To take advantage of price differentials at different times fordifferent sources of energy, it is necessary to control how much of eachenergy source supplies the load. In the exemplary control scheme thepower flow can be controlled accordingly.

Corrected elsewhere, the DC voltage will now stay stable and the powerflow change will be done by each slave inverter varying its PWM patternbased on the current slave signal multiplied by (x) a proportioningsignal coming from the CanBUS. Note that this may include a proportiongreater than 100%, which will cause the slave to supply more power thanthe master.

It is instructive to return briefly to FIG. 2 for a description of thedistributed generation control strategies.

The energy sources for the on-site power 34, 35 can come from manydifferent sources, but the most common are the burning of natural gas orsyngas, the use of variable speed engine generators or turbines, the useof fuel cells, and the use of solar power. Many of these sources requireDC/AC conversion.

In addition, energy storage technologies 38, 39 can help balance powerin the grid, smooth the peak generation requirements, and provideshort-term power during loss of other generation sources.

The power electronics and control scheme can take advantage of theability to run rotating machinery at variable speeds and convert theelectric power. Importantly, in a typical prior-art system, anyrotating-machinery power source such as a generator or alternator isrequired to run at some fixed fraction or multiple of the frequency ofthe local bus, and is required to maintain a fixed phase relationshipwith the local bus. But in the approach of FIG. 2, a power sourcedrawing upon rotating machinery is able to serve its purpose even if therotation is at some frequency and phase that is not linked to anythingabout the local bus. Saying this in a different way, there is norequirement that the rotating machinery be rotating at any particularfrequency, to be able to generate power to be supplied to the local bus26.

The approach according to the invention is thus a much more efficientapproach than the standard prior-art way of generating fixed-frequencyelectric power from rotating generators, especially over wide powerranges which are more necessary with smaller grids.

Similarly if a gas or steam turbine generator is used with a systemaccording to the invention, it becomes possible to eliminate a gearbox.This decreases size and allows variable speeds, which increasesefficiency. Size and weight issues can be particularly important indense areas where construction space is limited.

Returning to FIG. 3, it is noted that the DC/AC or DC/DC inverters showneach have a CanBUS connection 53 and a dedicated high-speed link 52 asshown.

The main redundant feature of the inverter controls is to be able tokeep running if one unit fails. If the master unit fails, the next slaveunit down the line becomes the master and continues on. So for example,if controller 47 is the master and if the other controllers are slaves,a provision must be made for the possibility that the controller 47 (orits inverter 46) may fail. In that case, an arbitration mechanism isemployed to promote one of the other controllers 45, 49, 51 to “master”status.

A series of alarm and warning messages can be sent via ETHERNET 55 overthe internet to remote monitoring facilities. This enables remotediagnostic capabilities and the ability to more quickly dispatchnecessary maintenance support.

Under this arrangement according to the invention, since any singleinverter or energy source can fail without causing complete gridfailure, then the faster the failure can be fixed, the less the chanceof a grid failure due to a second equipment failure.

Depending upon the particular grid and design and cost considerations,it may be required that the grid run at a reduced load until repairs canbe completed. This may be carried out using demand response signals fromthe system level controller 54 to an intelligent load shedding control,but done in such a way that the AC voltage on the grid always stayswithin specification.

In the event of a failure of the system level controller 54, theinverters 44, 46, 48, 50 are programmed to go into a default mode andkeep supplying the grid with balanced or pre-set proportions from thevarious energy sources. An alarm will be sent via ETHERNET 55 or simplythe absence of the required signal will trigger an alarm upstream.Again, this is done seamlessly.

These features cover all the significant faults that could stop theindependent grid 26 from providing voltage within the specified limitsunder any single point of failure.

The final redundant feature is the ability to switch the whole systemoff and go to a bypass switch (25 in FIG. 2) to feed the load just as atraditional distribution system does. This would normally be donemanually for safety reasons.

The bypass feature is easy to integrate because the independent grid isdesigned to integrate into existing grids with very few changesrequired.

A system control scheme for a single independent grid will now bedescribed.

To both balance thermal energy requirements and take advantage of pricedifferentials at different times for different sources of energy, it isnecessary to control how much of each energy source supplies the load ata given moment.

In the control scheme according to an embodiment of the invention, thisis done by sending a mathematical multiplier to each inverter 44, 46,48, 50 via CanBUS 53, such that it can be multiplied inside the slaveunits to the proportioning signals coming from whichever inverter isserving as the master inverter at that time.

Two-way communication via CanBUS 53 is used to adjust the amount ofenergy from different sources in the event of an inverter failure. Theinverters will automatically re-assign master control to the next slaveunit, but the total amount of energy fed to the independent grid must bemaintained.

The system may also include intelligent switchgear on certain loads toenable fast load shedding of less critical loads in the event ofdisruptions from an energy source causing total load capacity problems.

The communication to a central station for optimizing energy use andmonitoring system conditions is done via ETHERNET link 55 to theinternet.

As may be appreciated from the above discussion, what is describedincludes the application of a control scheme to control multipleinverters in order to drive an independent electrical grid fed bymultiple energy sources, controlling the energy flow from the varioussources, along with redundant back-up capabilities.

A main purpose behind creating such a control scheme is to facilitatethe efficient integration of distributed energy resources (DER) into theexisting power grid, without being limited to existing penetration levellimits imposed by operators of the existing grid. This ability tointegrate unlimited amounts of various sources of electrical energy,without regard to the present state of the existing grid, is at theheart of the need for this technology application.

The main way in which this task is accomplished is by having theIndependent Grid (feeder 14) be seen by the main grid 12 as aload-reduction type device, as opposed to a parallel interconnectedgenerator.

For projects requiring capacity larger than a single grid can support,the independent grids 14 are capable of being interconnected with otherindependent grids, for unlimited expandability.

The system provides for the independent grid and the efficient use ofdistributed generation assets, while overcoming the main challenges ofintegration with respect to control and stability of the existing grid.

Additional control features to improve redundant capability. TheMultiple Inverter Control Scheme can include a redundant, alwayson-line, back up PWM generator controller. If the Master PWM healthysignal is lost, the voltage source inverters can switch to the backupwithout interruption or delay.

There are other software features to command the voltage sourceinverters and energy source converters to behave in certain ways undercertain conditions to prevent complete system trips. The system may alsoinclude intelligent switchgear on certain loads to enable fast loadshedding of less critical loads in the event of disruptions from anenergy source causing total load capacity problems.

Upstream grid fault current limiting. In a Distributed Generationapplication with the inverter tied in parallel to a larger grid, thepath of power flow during an upstream grid fault is difficult topredict.

The main grid operator will want to shut down all connected distributedgeneration sources as quickly as possible so that the existingprotection breakers do not see an increase in the amount of faultcurrent that they must interrupt.

By simply measuring current, the inverter controller cannot tell thatsuch an upstream fault has occurred due to the various loads (such asinduction motors) and other generation sources which may create resonantcirculating paths for the current. The only way to sense an upstreamfault is through voltage measurement and comparison between the phases.

The inverter controller can be programmed to measure such anomalies andshut down on the next 200-microsecond transistor switch. The controllermust have the capability to filter noise from the measurements such thatit does not trip on nuisance events.

High Speed current control/power flow control. As noted above, with theinverter switching 40+times per half cycle, the ability to controlcurrent is much faster than any other means of standard electricalcontrols.

The inverter controller monitors the AC voltage and switches ON and OFFthe transistors to create current flow. The controller monitors thefeedback of the current sensor and can adjust the switching of thetransistors for both amplitude and phase angle of the current relativeto the voltage.

The control bandwidth is a function of the transistor switchingfrequency, with the feedback sampling frequency at a higher rate forimproved accuracy.

When the inverter is tied to an infinite grid, it is always in currentcontrol mode. If the grid is unstable, the inverter can help stabilizeit by monitoring the frequency and voltage and adjusting real orreactive current, or both, as required.

The inverter will have some inherent harmonic compensation for the maingrid due to the low source impedance of the current source. Activeharmonic filtering through current control of AC voltages measured atmultiple frequencies of the fundamental can be incorporated, but this isa significant extra feature and will require switching frequencieshigher than 5 kHz for harmonics beyond 5th and 7th.

Output AC voltage source operating mode. If the inverter is NOTconnected to an existing infinite grid, it can act as the voltagesource. In this mode it will turn the transistors ON and OFF and measurethe output voltage, adjusting to keep an accurate 60 Hz 480V sinusoidalwaveform. The current draw will be dependent upon the load, with thecurrent sensors acting as protection devices. The inverter may beprogrammed to lower the output voltage under heavy current draw ifdesired.

The inverters can be placed in parallel to increase the capacity of theoverall output. With special controls, multiple inverters can be groupedtogether to act as one, even though the DC link sources are notconnected together.

FIG. 4 shows detail of an inverter.

The diagram shows a simple circuit of 6 transistors including 61, 62connected in 3 series pairs between a DC supply 63 through an inductorto an AC line 58. The output AC current and voltage are measured onlines 59 and fed back to the inverter controller 60. Thus there iscurrent and voltage feedback as to all 3 phases of the AC line.

The basic concept of the power flow control is that either the toptransistor 61 or bottom one 62 is switched ON (depending upon the ACwaveform polarity at the time) to create a path from the AC voltagethrough the inductor to the + or − DC link, causing current to flow andenergy to be stored in the magnetic field of the inductor.

When this transistor turns OFF, the energy stored will be dumped throughthe freewheel diode of the opposite transistor into the DC linkcapacitors.

The timing of the transistor firing can be manipulated such that theamplitude of the current and phase angle of the voltage and current canbe controlled. This allows for separate real and reactive currentcontrol, current limits and with 180 degree phase shifts, the directionof the net power flow.

Fast Response. Due to the switching frequency of the transistors (5kHz), changes can be made every 200 uSeconds, or 41.5 times in each halfcycle (at 60 Hz).

FIG. 6 shows a prior-art standard interconnection. Power is generated bya utility company at generators 101, 102, each linked by a transformer103, 104 to a 230 kilovolt (for example) transmission line 106. This isstepped down by transformers 107, 108 to a 115 kilovolt transmissionline 109. This power may be stepped down by a transformer 110 to a 13.8kilovolt distribution line 111. The power may also be stepped down by atransformer 114 to a 13.8 kilovolt distribution line 115. Thetransmission lines may be any of several voltages including 230 kV or345 kV or 500 kV.

Distribution line 115 may serve buildings 116, 117, and 118, each havinga respective transformer 119, 120, 121. Within each building is arespective AC power bus 139, 140, 141.

A single building 118 may have a generator 123 and an inverter-basedlocal source of generated power 124. AC power from these sources iscarried to the building 118 by means of a bus 125. As will beappreciated from the above discussion, the regulatory environment willtypically place a strict and low limit on the amount of power that maybe passed from bus 125 back into the utility grid at 115, 109, and soon.

Importantly, the transformer connections are each, by definition,synchronous as between the primary and secondary windings thereof. Thismeans that the generator 123 is strictly required to be synchronous withthe utility grid and with the utility generators 101, 102. Likewise theinverter 125 is strictly required to be synchronous with the utilitygrid and with the utility generators 101, 102.

FIG. 7 shows an installation with an independent grid for a singlebuilding 126. The 115 kilovolt transmission line 109 is seen, just as inFIG. 6. Distribution line 115 is seen, as in FIG. 6. Building 129functions as in FIG. 6. Importantly, however, in FIG. 7 it may be seenthat building 126 is quite different. Utility power is passed throughtransformer 121 to line 127, where it is rectified at 128 and providedto DC bus 129. In this example a power source 130 provides power to aDC/DC converter 131 to the DC bus 129. Also in this example abidirectional DC/DC converter 133 connects a DC energy storage device132 with the DC bus 129. The energy storage device 132 might, forexample, be a battery.

What is important here is that the direction of the power flow is FROMthe grid not TO the grid, thus the independent grid is not seen by theexisting grid as a generator but as a load, and in this case, a reducedload, due to the on-site generators providing much of the energyrequired for the building.

A generator 134 provides power to rectifier 135, and from there to theDC bus 129. The DC power at the DC bus 129 is inverted at 126 and isprovided via line 137 to internal AC bus 138 to power loads in thebuilding 126.

Importantly, it will be appreciated that there is no requirement thatthe generator 134 be synchronous with anything else.

FIG. 8 shows an installation 160 with multiple independent grids, aswell as an unsychronized feeder 153. FIG. 8 shows many of the samefunctional blocks as FIG. 7. New functional blocks in FIG. 8 includecoupler 155 which couples the DC bus 129 with the feeder 153 (perhapsthrough a transformer 154 as shown). Building 144, line building 126,has an internal bus 146 that is not tied metallically to the buildingfeed at 142. In this way building 144 is similar to building 126, whichlikewise has an internal bus 138 that is not tied metallically to thebuilding feed at 121.

Note that in an exemplary embodiment, the feeder is a distribution levelvoltage, 13.8 kV. The power conversions take place at 480V, but anydistance will be required to have 13.8 kV lines. Note that this is alsothe highest voltage that can be used with the generator, in this case a10 MW turbine.

Associated with building 144 is a local DC bus 150, much as building 126has associated with it a local DC bus 129. This permits building 144 todraw power from the feeder 153 as desired, thereby perhaps making use ofthe power from sources 130, 132, or 134.

In this example there is a waste treatment plant 162, with a gas turbine161. The turbine 161 turns generator 163, which supplies AC power (inthis example, at 13.8 kilovolts) to the feeder 153. In this way eitheror both of buildings 144, 126 is able to draw upon the power from thegenerator 163.

To the extent that coupler 155 serves as an inverter (delivering powerfrom DC bus 129 to feeder 153), the coupler 155 is required to besynchronous with generator 163. But neither coupler 155 nor generator163 is required to be synchronous with the utility grid at 113 and 115.

Note that Inverter 155 is a three-wire current source, and itsynchronizes to the voltage source of the 10 MW generator. It can causecurrent to flow, and control the phase angle of this current to produce+/− kW or VAR depending on the phase angle. A zero degree shift is kW, a90 degree shift is VAR, and in between is a vector sum of the two.Inverter 128 is simply programmed not to allow −kW phase angles ofcurrent to flow.

Any feeder such as feeder 153 necessarily has exactly one power sourcethat defines the voltages on the feeder (thus called a “voltagesource”). Any other power sources that also deliver power to the feeder153 are necessarily not voltage sources but are mere current sources.Thus in a typical arrangement the inverter 155 (e.g. in FIG. 8) servesas a current source and not a voltage source.

Alternatively, however, it might develop that the generator 163 might goout of service. If this were to happen, then some other power sourcesuch as inverter 155 could be reconfigured to serve as the voltagesource for the bus 153. In this way, power could be delivered (see FIG.8) from the microsite containing power sources 121, 130, 134 to adifferent microsite such as that of building 144 even if generator 163were out of service.

Inverter 136 is a four-wire voltage source. It outputs three singlephase voltages between phases A, B, C and N. It does this no matter whatthe load, up to the silicon limit of the transistors so long as the DCcan be maintained within limits by whatever energy sources are feedingor taking from it.

With further reference to FIG. 8, many advantages and benefits of theinvention may be appreciated.

Consider, for example, the waste treatment plant 162. In a prior-artsystem, such a waste treatment plant 162 might be nearly unusablebecause of regulatory and engineering constraints imposed by the utilitycompany providing power at 113. In the arrangement shown in FIG. 8,however, power from the waste treatment plant 162 can pass throughtransformer 152 and electronics 151 to a local DC bus 150 associatedwith building 144. Similarly, power from the waste treatment plant 162can pass through transformer 154 and electronics 155 to a local DC bus129 associated with building 126. Nothing about this system requiresthat the power from the waste treatment plant 162 be “utility grade”. Itcould be inaccurate in its frequency or phasing, for example, ascompared with the utility grid at 113. If the power from the plant 162is three-phase power (as it is very likely to be in most embodiments),it is no problem at all if there are unbalanced loads somewhere on thefeeder 153. Even if there are unbalanced loads, the rectified powerpassed to DC bus 150 or 129 is capable of being put fully to use.

In FIG. 8, the electronics 128 may be configured so that the power flowis solely from left to right in FIG. 8. If so, then on a practical levelthe system that is to the right of electronics 128 will appear to theexisting utility grid as a load reduction and not as an interconnectedgenerator.

As mentioned above, in an exemplary embodiment the plant 162 with itsgenerator 163 might well not be generating utility grade power. Forexample the power generated by the generator 163 may sometimes be ofslightly higher frequency relative to specifications, and may at othertimes be of slightly lower frequency. There might also be unbalancedloads somewhere on the feeder 153 as between the three phases. Therecould also be poor-power-factor loads somewhere on the feeder 153,giving rise to a condition of the voltage and current on the feeder 153being pulled out of phase with each other. In the face of all of thesepossible degradations in the quality of the power supplied on the feeder153, it is possible to configure inverter 155 (together with local DCbus 129 and energy storage 133, 132) to bring about substantialimprovements in the power quality on the feeder 153, as will now bediscussed.

In an exemplary power-quality-enhancement approach, inverter 155 drawspower from feeder 153 from time to time, some of which is stored inenergy storage 133, 132. The power drawn need not be power of highquality since it will be rectified anyway on its way to storage 133, 132(passing over DC bus 129). Inverter 155, however, also monitors theinstantaneous voltages present on the three (typical) phases of thefeeder 153. If inverter 155 notices an imperfect voltage waveform on oneof the phases, including timing issues of the waveform that diverge fromthe desired frequency regulation, then it nearly instantaneously pumpssome power into that phase (or draws power from that phase) so as tobring the waveform closer to the ideal. The availability of power on thedesired time scale—far faster than the reaction time of the mechanicalthrottle on generator 134—is achieved through the energy storage device132. This activity by the inverter 155 is carried out upon each of thethree phases and can result in the effective waveforms on the phases ofthe feeder 153 being utility-grade quality.

In this arrangement, the generator 163 serves as a voltage source, whileinverter 155 serves as a three-wire power source (or sink) for thepurpose of dynamic frequency stabilization.

The power quality issues that are voltage related can be solved byvarious forms of VAR control or harmonic filtering, but that will notfix a frequency problem, which can only be fixed by changing net kW flowfrom an energy source. It is not correct to say that the inverter 155sinks or sources current (which a D-VAR does as well); it sinks orsources power (kW), which it gets or sends from the energy storagesource.

On a practical level this could mean, among other things, that the powerprovided to other end users on the feeder 153 can be sold at a pricethat is appropriate for utility-grade power (typically a retail price).In contrast, had the generator 163 been connected in a prior-art fashiondirectly to a utility grid such as at 115, the power would only be ableto be sold at much lower wholesale prices. Said differently, theavailability of the hardware 155, 129, 133, 132 together withappropriate configuration permits providing frequency stability so thatnon-utility-grade power is rendered utility-grade, offering economicbenefits.

Returning now to FIG. 8, another benefit may be seen. Consider theeffect upon building 144 or 126 if the utility grid 113, 114, 115 islost to the end users at buildings 144 and 126. In a prior-artarrangement, any local backup power generation is permitted to beconnected to the building only by means of a “transfer switch”. Thetransfer switch connects either the utility grid 115 to the building bus146, or the local backup power such as from generator 134 or generator163. With such an arrangement, any switching of the transfer switch willresult in a disruption of power.

Some transfer switches require tens or hundreds of milliseconds toswitch. But another issue is that some utilities require that thecircuit “go dark” for at least hundreds of milliseconds, before thelocal backup power comes on line. As such, this is often a regulatory,not technical, limitation. But as mentioned above, the systems accordingto the invention do not suffer from such interruptions.

In contrast, however, with an arrangement 160 as shown in FIG. 8, a lossof the utility grid 115 need not result in any disruption at all of thepower to the end user AC buses 146, 138. The loss of utility powermerely means that electronics 148, 128 are then unable to provide DCpower to the local DC buses 150, 129 respectively. But other sources ofDC power permit continued and uninterrupted AC power through inverters149, 136 to local AC buses 146, 138.

Returning again to FIG. 8, it will be appreciated that electronics 155,128, 151, 148 can be configured to respond to out-of-band signaling thatpermits taking action based upon price changes among the various sourcesof electrical power. If a particular power source becomes moreexpensive, the electronics can draw less power from that source. If aparticular power source becomes less expensive, then the electronics candraw more power from that source.

Any electrical power distribution system will have fault protectiondevices which are intended to open in the event of a fault, so as toisolate a failed portion of the system from other non-failed portions ofthe system. What is undesirable, however, is if in the event of a fault,the fault currents fail to rise to a level that suffices to trigger therelevant fault protection device. With appropriate configuration,however, the inverters feeding a given feeder or bus or grid can respondto internal control logic to sense voltage anomalies on the feeder orbus or grid; when this happens the inverters can supply extra current soas to help clear the relevant fault protection devices as quickly aspossible.

CONCLUSION

The advantages of this control scheme, over other ways to perform thesharing of multiple power sources, include: [0144] A lower output sourceimpedance requirement for the inverters since impedance is not requiredto be added to aid active power sharing. This reduces cost andefficiency loss, and increases the amount of available fault currentthat can be sourced from the inverters. [0145] A simple control schememay be employed, with no need for active Micro-grid type controls.[0146] It is possible to retain redundant operation with nosingle-component failure able to cause a complete shutdown.

While the invention has been described with respect to particularembodiments, the invention is not limited thereto. Those skilled in therelevant arts will have no difficulty devising myriad obviousimprovements and variations, all of which are intended to be within thescope of the claims which follow, when properly construed.

1. A system for use with AC power at a frequency, the frequency defininga period, the system comprising: an AC feeder; a control path, thecontrol path being separate from the AC feeder, the control pathcommunicating control information with a latency no greater thanone-tenth the period of the AC power; at least two inverters, eachinverter receiving DC power from a respective DC power source, eachinverter having an AC output metallically connected to the AC feeder,the metallic connection free of any electrical transformer, therespective DC power sources isolated from each other; each inverterdisposed in a first state to generate AC power unsynchronized with anyother AC phase, and in said first state to measure its AC output phaseand to communicate that phase external to the inverter by means of thecontrol path; each inverter disposed in a second state to receive an ACphase signal from external to the inverter by means of the control path,and to generate AC power synchronized with the received AC phase signal,said AC power provided to the AC feeder; control means responsive to apredetermined event causing the first one of the at least two invertersto take said first state and causing any other inverters to take saidsecond state; said means after said predetermined event responding tofailure of the first one of the at least two inverters by causing aninverter other than the first one of the at least two inverters tochange to said first state; said changes of state occurring within aninterval less than one-half the period of the AC power; the AC feederhaving loads drawing AC power therefrom.
 2. The system of claim 1wherein the number of inverters is at least three.
 3. The system ofclaim 1 wherein each inverter generates wye three-phase electricalpower.
 4. A method for use with a power system, the power systemcomprising a utility grid providing AC electric power connectivity andextending geographically to a plurality of first end-user locations andto a plurality of second end-user locations, the first end-userlocations comprising more than one and less than all of the end-userlocations of the utility grid, each of the first end-user locationscomprising a local bus providing AC electric power connectivity, eachlocal bus having associated with it at least one AC load metallicallyconnected thereto and drawing power therefrom, each local bus coupled bymeans of a respective first at least one coupler to the utility grid,the at least one coupler comprising a first power-supply-inverter and asecond power-supply-inverter, each power-supply-inverter having an ACinterface and a DC interface and a controller, eachpower-supply-inverter disposed in a first mode to receive DC powerreceived at the DC interface and to generate AC power delivered at theAC interface, and disposed in a second mode to receive AC power receivedat the AC interface and to generate DC power delivered at the DCinterface, the DC interface of the first power-supply-inverterelectrically connected to the DC interface of the secondpower-supply-inverter, whereby the at least one coupler has a first ACinterface defined as the AC interface of the first power-supply-inverterand a second AC interface defined as the AC interface of the secondpower-supply-inverter, the controller of the first power-supply-inverterand the controller of the second power-supply-inverter coupled so as toprevent the first power-supply-inverter and the secondpower-supply-inverter from being in the first mode simultaneously forextended intervals, and so as to prevent the first power-supply-inverterand the second power-supply-inverter from being in the second modesimultaneously for extended intervals, each power-supply-inverterdisposed when in its first mode to generate its AC power delivered atits AC interface consistent in voltage and phase and frequency with anyAC power present external to said AC interface, the power system furthercomprising a feeder providing AC electric power connectivity andextending geographically to the plurality of first end-user locations,each local bus coupled by means of a respective second at least onecoupler to the feeder, the feeder being unsynchronized with the utilitygrid, the method comprising the steps of: at a first time, powering eachof the local buses by means of a respective coupler drawing power fromthe utility grid; at a second time, generating locally generatedelectrical power at one of the first end-user locations, and passingsome of said locally generated electrical power to others of the firstend-user locations by means of the feeder.
 5. A method for use with asystem, the system for use with AC power at a frequency, the frequencydefining a period, the system comprising an AC feeder; a control path,the control path being separate from the AC feeder, the control pathcommunicating control information with a latency no greater thanone-tenth the period of the AC power; at least two inverters, eachinverter receiving DC power from a respective DC power source, eachinverter having an AC output metallically connected to the AC feeder,the metallic connection free of any electrical transformer, therespective DC power sources isolated from each other; each inverterdisposed in a first state to generate AC power unsynchronized with anyother AC phase, and in said first state to measure its AC output phaseand to communicate that phase external to the inverter by means of thecontrol path; each inverter disposed in a second state to receive an ACphase signal from external to the inverter by means of the control path,and to generate AC power synchronized with the received AC phase signal,said AC power provided to the AC feeder; the AC feeder having loadsdrawing AC power therefrom; the method comprising: at a first time,causing the first one of the at least two inverters to take said firststate and causing any other inverters to take said second state; inresponse to failure of the first one of the at least two inverters,causing an inverter other than the first one of the at least twoinverters to change to said first state, said changes of state occurringwithin an interval less than one-half the period of the AC power.
 6. Asystem comprising: a three-wire AC power grid; at least one four-wirewye inverter connected to the AC power grid and serving as a voltagesource therefor, the at least one four-wire wye inverter having arespective first power output, any power on the AC power grid having apower quality, the power on the AC power grid defining a voltage andfrequency for each phase thereof; a three-wire inverter connected to theAC power grid and serving as a current source therefor, the three-wireinverter having a respective power output; the four-wire inverterdisposed to sample the voltage on each phase of the AC power grid, andto source or sink power on each phase of the AC power grid so as todynamically maintain frequency and voltage regulation and improve thepower quality of the power on the AC power grid.
 7. A system comprising:a three-wire AC power grid; at least one four-wire wye inverterconnected to the AC power grid and serving as a voltage source therefor,the at last one four-wire wye inverter having a respective first poweroutput, any power on the AC power grid having a power quality, the poweron the AC power grid defining a voltage for each phase thereof; aninduction generator connected to the AC power grid and serving as acurrent source therefor, the induction generator having a respectivepower output; the four-wire inverter disposed to sample the voltage oneach phase of the AC power grid, and to source or sink power on eachphase of the AC power grid so as to dynamically maintain frequency andvoltage regulation and improve the power quality of the power on the ACpower grid.
 8. A method for use with a system comprising a three-wire ACpower grid, at least one four-wire wye inverter connected to the ACpower grid and serving as a voltage source therefor, the at least onefour-wire wye inverter having a respective first power output, any poweron the AC power grid having a power quality, the power on the AC powergrid defining a voltage for each phase thereof, an additional AC currentsource connected to the AC power grid and serving as a current sourcetherefor, the additional AC current source having a respective poweroutput, the method comprising the steps of: at the four-wire inverter,sampling the voltage on each phase of the AC power grid, and sourcing orsinking power on each phase of the AC power grid so as to dynamicallymaintain frequency and voltage regulation and improve the power qualityof the power on the AC power grid.
 9. A system comprising: a three-wireAC power grid; a generator connected to the AC power grid and serving asa voltage source therefor, any power on the AC power grid having a powerquality, the power on the AC power grid defining a voltage for eachphase thereof; a four-wire wye inverter connected to the AC power grid,the four-wire wye inverter also connected to a DC bus, the DC bus havingDC power storage connected thereto; the four-wire inverter disposed tosample the voltage on each phase of the AC power grid, and to source orsink power on each phase of the AC power grid so as to dynamicallymaintain frequency and voltage regulation and improve the power qualityof the power on the AC power grid.
 10. A method for use with a systemcomprising a three-wire AC power grid, a first generator connected tothe AC power grid and serving as a voltage source therefor, any power onthe AC power grid having a power quality, the power on the AC power griddefining a voltage for each phase thereof, a four-wire wye inverterconnected to the AC power grid, the four-wire wye inverter alsoconnected to a DC bus, the DC bus having DC power storage connectedthereto, the method comprising the steps of: at the four-wire inverter,sampling the voltage on each phase of the AC power grid, and sourcing orsinking power on each phase of the AC power grid so as to dynamicallymaintain frequency and voltage regulation and improve the power qualityof the power on the AC power grid.
 11. A method for use with a first busand a second bus, the first bus and the second bus connected by abreaker, and with a plurality of inverters, each inverter providing ACpower to the first bus at a first voltage, each inverter defining somemaximum current associated therewith, the method comprising the stepsof: sensing a fault on the second bus by means of voltage sensing; inthe event of a sensed fault, releasing each inverter to feed its maximumpossible current to the first bus.
 12. The method of claim 11 furthercomprising the step, performed after the releasing step, of detecting areturn of the first voltage, and thereafter, ceasing the feeding ofmaximum possible current to the first bus.
 13. A system comprising afirst bus and a second bus, the first bus and the second bus connectedby a breaker, and a plurality of inverters, each inverter providing ACpower to the first bus at a first voltage, each inverter defining somemaximum current associated therewith, each inverter responsive to asensed fault sensed by means of voltage sensing by delivering itsmaximum possible current to the first bus.
 14. The system of claim 13wherein each inverter is further responsive to detection of a return ofthe first voltage by ceasing the delivery of the maximum possiblecurrent to the first bus.
 15. An inverter, the inverter providing ACpower at a first voltage, each inverter defining some maximum currentassociated therewith, each inverter responsive to a sensed fault sensedby means of voltage sensing by delivering its maximum possible current.16. The inverter of claim 15 wherein the inverter is further responsiveto detection of a return of the first voltage by ceasing the delivery ofthe maximum possible current.
 17. A system comprising a first DC bus, asecond DC bus, a feeder, a first AC bus, a second AC bus, and first andsecond transformers each connected with the feeder; the system furthercomprising a first inverter coupling the first AC bus with the first DCbus, a second inverter coupling the second AC bus with the second DCbus; a third inverter coupling the first DC bus with the firsttransformer; a fourth inverter coupling the second DC bus with thesecond transformer; a first load connected with the first AC bus; afirst generator connected with the feeder, the first generator having apower output level; a second load connected with the feeder; a secondgenerator connected with the second DC bus and having a power outputlevel; the second load consuming power amounting to at least the poweroutput of the first generator; the first load consuming power amountingto at least a portion of the power output of the second generator.
 18. Amethod for use with system comprising a first DC bus, a second DC bus, afeeder, a first AC bus, a second AC bus, and first and secondtransformers each connected with the feeder; the system furthercomprising a first inverter coupling the first AC bus with the first DCbus, a second inverter coupling the second AC bus with the second DCbus; a third inverter coupling the first DC bus with the firsttransformer; a fourth inverter coupling the second DC bus with thesecond transformer; a first load connected with the first AC bus; afirst generator connected with the feeder, the first generator having apower output level; a second load connected with the feeder; a secondgenerator connected with the second DC bus and having a power outputlevel, the method comprising the steps of: generating power at the firstgenerator at a respective power output level; generating power at thesecond generator at a respective power output level; at the second load,consuming power amounting to at least the power output of the firstgenerator; and at the first load, consuming power amounting to at leasta portion of the power output of the second generator.
 19. A systemcomprising a first DC bus, a second DC bus, a feeder, a first AC bus, asecond AC bus, and first and second transformers each connected with thefeeder; the system further comprising a first inverter coupling thefirst AC bus with the first DC bus, a second inverter coupling thesecond AC bus with the second DC bus; a third inverter coupling thefirst DC bus with the first transformer; a fourth inverter coupling thesecond DC bus with the second transformer; a first load connected withthe first AC bus; a first generator connected with the feeder, the firstgenerator having a power output level; a second load connected with thefeeder; a second generator connected with the second DC bus and having apower output level; the second load consuming power amounting to atleast the power output of the first generator; the first load consumingpower amounting to at least a portion of the power output of the secondgenerator; the first and second AC buses not synchronized with eachother; the feeder not synchronized with either of the first and secondAC buses.
 20. A method for use with system comprising a first DC bus, asecond DC bus, a feeder, a first AC bus, a second AC bus, and first andsecond transformers each connected with the feeder; the system furthercomprising a first inverter coupling the first AC bus with the first DCbus, a second inverter coupling the second AC bus with the second DCbus; a third inverter coupling the first DC bus with the firsttransformer; a fourth inverter coupling the second DC bus with thesecond transformer; a first load connected with the first AC bus; afirst generator connected with the feeder, the first generator having apower output level; a second load connected with the feeder; a secondgenerator connected with the second DC bus and having a power outputlevel, the method comprising the steps of: generating power at the firstgenerator at a respective power output level; generating power at thesecond generator at a respective power output level, the power generatedat the second generator not synchronized with the power generated at thefirst generator; at the second load, consuming power amounting to atleast the power output of the first generator, the power consumed at thesecond load not synchronized with the power generated at the firstgenerator and not synchronized with the power generated at the secondgenerator; and at the first load, consuming power amounting to at leasta portion of the power output of the second generator.
 21. A systemcomprising a first DC bus, a second DC bus, a feeder, a first AC bus, asecond AC bus, and first and second transformers each connected with thefeeder; the system further comprising a first inverter coupling thefirst AC bus with the first DC bus, a second inverter coupling thesecond AC bus with the second DC bus; a third inverter coupling thefirst DC bus with the first transformer; a fourth inverter coupling thesecond DC bus with the second transformer; a first load connected withthe first AC bus; a second generator connected with the second DC busand having a power output level; the first load consuming poweramounting to at least a portion of the power output of the secondgenerator.
 22. A method for use with system comprising a first DC bus, asecond DC bus, a feeder, a first AC bus, a second AC bus, and first andsecond transformers each connected with the feeder; the system furthercomprising a first inverter coupling the first AC bus with the first DCbus, a second inverter coupling the second AC bus with the second DCbus; a third inverter coupling the first DC bus with the firsttransformer; a fourth inverter coupling the second DC bus with thesecond transformer; a load connected with the first AC bus; a generatorconnected with the second DC bus and having a power output level, themethod comprising the steps of: generating power at the generator at arespective power output level; at the load, consuming power amounting toat least a portion of the power output of the generator.